Vidicon target comprising infrared absorber



Dec. 8, 197()` R. P. ANAND ET AL Y 3,546,520

VIDICON TARGET `COMPRISING INFRARED ABSORBER Filed Dec. 20,*19'67 3 Sheets-Sheet l AT ORA/EV `Dec. 8, 19,70 R, P, ANAND ETAL 3,546,520 I 1 l vIDIcoN TARGET coMPRIsING INFRAREDA sonn.

Filed Dec. 20,1967 s sheets-sheet 2 I I l v2.7 s 2.9 .3.0 3./ 3.2 3.3 3.a //7 x /03 PER DEGREE /f 8, 1970 R. P. ANAND ETAL 3,546,520

VIDICON TARGET COMPRISING INFRARED ABSVORBER 3 Sheets-Sheet 5 Filed Dec. 20, 1967 NME FIG. 7

7. 0 9 w 7. a a. 0

WA VEL ENG 7H M/CRONS FIG.A 8

WITHOUT4 F/LTER WAVELENGTH A United States Patent O U.S. Cl. 313-112 2 Claims ABSTRACT F THE DISCLOSURE Photosensitive films, such as those used in the target of vidicon tubes, lose photoconductivity when exposed to intense illumination. The phenomenon is termed burn-in. It has been discovered that radiation having wavelengths greater than approximately 0.7;1. in the infrared region is harmful. The invention comprises positioning an appropriate infrared filter between the photosensitive film and the harmful light source in order to prevent harmful infrared radiation from being incident upon the photosensitive film. For example, the filter may be an external one removably affixed to the face of the vidicon tube or may comprise a layer of infrared reflecting material deposited on the exterior surface of the vidicon window. Alternatively7 the filter may be a thin film of an infrared absorbing material incorporated in the target structure of the vidicon tube.

BACKGROUND OF THE INVENTION This invention relates to the protection of photosensitive materials from loss of photoconductivity and more particularly to the protection of photosensitive targets for use in electron discharge devices.

It is known that in vidicon camera tubes photosensitive films (typically SbzSa, Bi2S3 and As2S3) when exposed to intense illumination, lose photoconductivity. This phenomenon is termed burn-in and may be either a temporary or a permanent effect.

Temporary burn-in is caused by steady illumination of a substantially constant intensity. The effect of such burnin is to reduce the signal to noise ratio inasmuch as the burn-in reduces the photoconductive sensitivity and increases the dark current. It appears that temporary burnin is the result of changes in the electron trapping spectrum of the photosensitive film. That is, the steady illumination incident upon the photosensitive films causes trapped electrons to leave the trapped level and thereby allows photoconductive electrons to fall from the conduction band into the empty trapped level. Consequently, the photosensitive film tends to have memory which produces the aforementioned effect. Temporary burn-in gradually anneals itself as the photoconductive electrons leave the trapped levels.

Permanent burn-in, on the other hand, is caused by photoflash illumination. The effect of such burn-in is substantial loss in photoconductivity of the portions of the photosensitive film exposed to the incident radiation. It appears that permanent burn-in results from the disassociation and recrystallization of the photosensitive film.

SUMMARY OF THE INVENTION It has been discovered that radiation in the near infrared range having wavelengths greater than approximately 0.7M causes both temporary and permanent burn-in.

Permanent burn-in is virtually eliminated in accordance with one aspect of the invention by positioning an infrared filter between the photosensitive film and the harmful light source. The filter could comprise a glass member appropriately doped with an infrared absorber, and removably afiixed to the face of the vidicon tube. Alternatively, an infrared absorbing or reflecting layer could be depositing directly on the target itself thereby forming a unitary structure.

Because the region of high photoconductive sensitivity of the film (eg. (15S/t for Sb-2S3) is outside the harmful range of wavelengths greater than. 0.7M, insertion of the infrared filter does not cause appreciable loss in sensitivity of the film.

Temporary burn-in, in comparison, is a time-integrated effect which also can be significantly reduced by positioning an infrared filter or reflector, as described above, between the photosensitive film and the harmful light source.

BRIEF DESCRIPTION OF THE DRAWING The above and other features of the invention, together with its various advantages, will be easily understood from the following more detailed description taken in conjunction with the accompanying drawings, in which:

FIG. l is a cross section of a vidicon camera tube in accordance with one embodiment of the invention;

FIGS. 2A, 2B and 2C show cross sectional views of target structures in accordance with several embodiments of the invention;

FIG. 3 shows the increase in dark current in a permanent burn-in area;

FIG. 4 is a graph of dark current versus target voltage;

FIG. 5 is a graph of dark current versus temperature;

FIGS. 6A and 6B show the increase in dark current and decrease in photoconductive responsive, respectively, immediately following exposure causing temporary burn- 1n;

FIG. 7 has a graph of transmission Versus wavelength for an infrared filter; and

FIG. 8 is a graph of photoconductive response versus wavelength.

DETAILED DESCRIPTION Turning now to FIG. 1, there is shown a vidicon camera tube 10 in accordance with one embodiment of the invention comprising an exhausted elongated, cylindrical glass envelope 11. One end of the envelope 11 is closed by a glass base 12 which accommodates connecting pins 13. These connecting pins are connected to various parts of an electrode system 14 which is mounted in the envelope 11. The electrode system 14, shown diagrammatically, comprises, inter alia, a cathode 15, a control grid 16 and a perforated anode 17, the latter being connected to a wall electrode 18. The electrode system 14 produces an electron beam 19 by which a target plate 20 at the other end of the envelope 11 is scanned.

The target plate 20 typically comprises a photosensitive material (eg. Sb2S3, Bi2S3 or As2S3) which has been deposited on a transparent, electrically conducting signal electrode 21, which extends over the inner window 22 formed by the right hand end of the envelope 11. The signal electrode 21 typically comprises a thin layer of tin oxide deposited on the Window 22.

A current supply conductor 23, extending through the wall of the envelope 11, is connected to signal electrode 21. A picture scene is projected, for example, by means of an optical system represented diagrammatically by a lens 24, through the window 22 and the signal electrode 21 onto the target plate 20. In order to obtain electrical signals corresponding to the picture projected, suitable voltages are applied to the electrodes of system 14; and by means of a voltage source 25, and via a signal resistor 26, the signal electrode 21 receives a voltage (eg. 40 volts) which is positive relative to the cathode 15. By means of the conventional deflection and focusing coils surrounding the tube and designated in common by 27, the electron beam 19 is made to scan the free surface of 3 the target plate 20. The light incident upon the target plate 20 produces a charge distribution which is representative of the picture being viewed. This charge which is neutralized by the scanning electron beam 19, produces an electrical signal that can be derived via a capacitor 28 from the signal resistor 26.

In accordance with one embodiment of the invention, in order to protect the photosensitive target plate 20 from both temporary and permanent burn-in, an infrared lter 30, mounted in a support 29, is positioned between the harmful light source and the target plate 20. The filter 30 typically comprises a glass member appropriately doped with an infrared absorbing material (e.g. iron oxide) which preferably absorbs infrared radiation having wavelengths greater than 0.7M.

Alternatively, as shown in FIG. 2A, a thin layer 31 of an infrared absorbing material (e.g. the oxides or nitrides of Ta, Nb or Ti) may be deposited on the inner surface of the window 22. The signal electrode 21 is then formed on the layer 31 and the photosensitive target plate 20 is deposited in turn on the signal electrode 21 by well-known vapor deposition techniques, for example. The layer 31 preferably absorbs infrared radiation having wavelengths greater than 0.7,u.

As shown in FIG. 2B, burn-in is reduced by coating the exterior surface of the window 22 with an infrared reflecting layer 32. Here, the layer 32 preferably reects radiation having wavelengths greater than 0.7tt.

It is also possible to reduce burn-in without fabricating a separate infrared absorbing or reflecting layer. As shown in FIG. 2C, the window 22 may be doped with an infrared absorbing material designated by 33. Again, the material preferably absorbs radiation having wavelengths greater than 0.7M.

Although the following discussion is directed to antimony-trisulfide lms, the results of course apply generally to other photosensitive films (e.g. Bi2S3, AszSa).

PERMANENT BURN-IN The experimental results of permanent burn-in are shown in FIGS. 3 through 5. Burn-in was produced by exposing a photosensitive antimony-trisulfide film to photoflash illumination produced by a flash lamp rated at 45 watt-seconds, 1/1200 second. The spectral distribution of the lamp corresponds approximately to a temperature of 40005000 K. The distance between the vidicon lens and the lamp was varied from 60 to 160 cm.

After permanent burn-in was produced in the antimonytrisulde film by photoash illumination, the film was scanned with a line monitor and the dark current output of the lms recorded for a typical target voltage (e.g. 10 volts) 'as shown in FIG. 3. As the film was scanned the dark current registered a normal Value of about l na., but as the burnt-in spot was reached, the dark current increased rapidly to about 100 na. indicating that the film had been damaged. FIG. 4 shows the increase in dark current of the burnt film for a range of target voltages from to 50 volts. The temperature dependence of the dark current is shown in FIG. 5. The characteristic for the burnt film shows both an increase in dark current output and an increase in the slope of the characteristic.

When a 2 mm. Schott KG-3 filter was positioned between the flash lamp and the vidicon lens at distances varying between 60 and 160 cm., no burn-in was detected. When instead a filter with a sharp cutoff from 0.4, to 0.6;1. was used, burn-in was produced.

TEMPORARY BURN-IN The experimental results of temporary burn-in are shown in FIGS. 6 and 7. To produce burn-in the antimony-trisulde film of a commercial vidicon was illuminated at different times with three types of light sources: (1) a tungsten lamp, (2) a He-Nc laser and (3) a calibrated tungsten-iodine lamp.

A 60 watt frosted tungsten lamp with a spherical reector was placed 152 cm. from the vidicon target. The light output of lamp was focused on the target through an f/ 1.5 lens. 'Ihe target was exposed to steady illumination from the lamp for a period of about two minutes. Immediately thereafter the target was scanned with a line monitor `and the results recorded as shown in FIGS. 6A and 6B. As a line was scanned the normal values of dark current (e.g. l0 na.) and photosensitivity (e.g. 0.080 microamp./microwatt) were recorded. When, however, a burnt-in spot was scanned, the dark current increased to nearly 1l na. and the photosensitivity decreased to about 0.070 microamp./microwatt as shown in FIGS. 6A and 6B, respectively.

Thirty minutes after the illumination was removed, both dark current and photosensitivity returned to their normal values indicating that the burn-in produced was in fact temporary.

Next, Schott KG-3 infrared absorbing filters of different thicknesses were interposed between the light source and the vidicon target. Filters of 2 mm., 4 mm. and 7 mm. were used and in each case the exposure was for a period of two minutes. With the 2 mm. and 4 mm. filters burnin was produced, but with the 7 mm. filter no burn-in was observed after two minutes of exposure.

The absorption curves of these `filters is shown in FIG. 7. The passband of the filters is the range from about 0.3/.t to 0.7,tt. Below 0.3M and above 0.8,@ the transmission decreases rapidly to less than 0.01.

The spectral photoconductive response of the antimony-trisulde target with and without these filters is shown in FIG. 8. The response was measured with a calibrated light source having a color temperature of 30'00o K. FIG. 8 shows that the maximum decrease in photosensitivity is only 20% and occurs at 0.6,u.. At 0.55/t, the region of high photosensitivity for antimony trisulfide, the decrease in photosensitivity is only about 12%.

In addition, no burn-in was produced when the target was exposed for periods of several minutes to a 1.0 mw. He-Ne laser at 0.6328/r.

Next, the target was exposed to light in the 0.4M to 0.7M range using a monochromator and a calibrated light source at 3000 K. The distance between the light source and the monochromator was l2 om. and the distance between the vidicon lens and the monochromator was 60 cm. No bum-in was observed with exposures of l0 seconds to l0 minutes for various passbands having a 6510 A. half width.

From these observations it is concluded that temporary burn-in is associated with steady state illumination results from radiation in the near infrared beyond 0.7M. The effect appears to be time-integrated and reversible.

It is to be understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which can be devised to represent application of the principles of the invention. Numerous and varied other arrangements can be devised in accordance with these principles by those skilled n the art without departing from the spirit and scope of the invention.

What is claimed is:

1. An electron discharge device for converting visible images into electrical representations thereof and being disadvantageously subject to burn-in comprising, in combination,

an envelope having a window transparent to visible radiation,

a source of electrons disposed at one end of said envelope,

a photosensitive film disposed adjacent said window,

said film being directly responsive to visible radiation from an external light source, having high photoconductive response to said visible radiation from said Source and being disadvantageously subject to burn-in by infrared wavelengths of said radiation greater than approximately 0.7M,

means for projecting the electrons in the form of a beam from said electron source to said photosensitive film,

means for scanning said beam across said photosensitive film,

means for deriving from said photosensitive film an electrical signal indicative of the intensity of the external visible radiation, and

means for preventing burn-in in said photosensitive film comprising means for preventing infrared radiation of wavelength greater than approximately 0.7M from being incident upon said photosensitive film, said infrared preventing means being highly transmssive to visible radiation and said photosensitive film being substantially unresponsive to thermal or electrical changes in said infrared preventing means said latter means comprising a layer of infra-red absorbing material deposited on one surface of said window, said material being selected from the group consisting of the oxides and nitrides of tantalum, niobium, and titanium.

2. The device of claim .1 wherein said photosensitive lm comprises a material selected from the group consisting of antimony trisulde, bismuth trisulde and arsenic trisulfide.

References Cited UNITED STATES PATENTS ROBERT SEGAL, Primary Examiner U.S. Cl. XR. 

